Adds deprecation note on gen-kubectldocs README and Makefile

[Nycto-c05]

Add deprecation note on gen-kubectldocs

Follows
Deprecate -> Announce -> Wait -> Remove (if needed)

Partially Addresses (Sub-task: 2) #448

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[tengqm]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Nycto-c05, tengqm

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